Pavement materials, such as soil, sand, aggregate, asphalt, and cement, typically require quality control testing during the construction process for physical attributes such as density, modulus, cement content, and moisture. The moisture and density relationship is an important property that is monitored during road construction and rehabilitation. In order to provide durable roads, the soil base layer and all hot mixed asphalt layers above it are compacted to density values that are specified in the engineering design. Destructive tests and nondestructive tests are used throughout the industry for quality control of these materials.
In laboratory tests, cylindrical samples are prepared, typically with a gyratory compactor, and various material properties are studied to determine the best mix design for a pavement. In field destructive tests, cylindrical samples are cored from test strips, newly constructed roads, or existing roads. The material properties of these samples are then used to evaluate whether the test strip or the new pavement meets the design criteria and whether the existing road is in good operating condition or in need of repairs.
Currently, several methods are used for measuring the density of cylindrical asphaltic samples including dimensional analysis, the water displacement method, and the paraffin-coated or para-film-covered method. In each case, the bulk density of a sample is derived by, as in the definition, dividing the dry sample mass by the estimated sample volume. All methods require a balance with a sensitivity of 0.1 gram to measure the mass of the sample.
In the dimensional analysis method, sample volume is determined from radius and thickness (height) measurements of the sample. Here, many readings of radius and thickness of the sample are made either manually using a vernier caliper or automatically using a laser system. The average values of radius and thickness are then used to calculate the sample volume. Other methods use the Archimedes Principle related to water displacement for determining the sample volume. These methods require a large container filled with clean water wherein the water temperature is monitored and controlled at a specific temperature, e.g. at 25 degrees Celsius. The sample is kept immersed in water for approximately 4 minutes during the test and the weight of the sample, while suspended in water, is recorded. In the “paraffin-coated” method, after determining the dry weight of the sample, a thin coating of paraffin is applied to cover the entire surface area of the sample. Then, the sample is weighed again in air. Finally, the sample is weighed while immersed in water. More details can be found in standards ASTM D 2726 for the water displacement method and ASTM D 1188 for the paraffin-coated method.
In the field, at present, the moisture content and density of materials are typically determined using two non-destructive test methods. One method uses radioactive materials or nuclear gauges and is commonly known as the “nuclear method”. The other method uses the electrical response of the material without radioactive materials, using electromagnetic devices, and is commonly known as a “non-nuclear” method or the “electromagnetic method”.
Nuclear radiation gauges, such as those described in U.S. Pat. Nos. 2,781,453 and 3,544,793 have been widely used for measuring the density of soil and asphaltic materials and have been in use in the road construction industry since the 1950's. Such gauges employ a nuclear radiation source, typically a mono-energetic source, which directs gamma radiation into the test material, and a radiation detector, typically a Geiger Mueller tube, located adjacent to the surface of the test material for detecting radiation scattered back to the surface. The gamma radiation interacts with matter in the test material, either by losing energy and changing direction (Compton interactions) or by terminating (photoelectric interactions). Consequently, the gamma radiation detected by the radiation detector has a continuous energy spectrum. From this detector reading, the density of the material can be determined.
These gauges are designed to operate both in a “backscatter” mode and in a direct transmission mode. The radiation source is vertically moveable from a backscatter position where it resides within the gauge housing (e.g., the nuclear gauge rests on the surface of the pavement or soil) to a series of direct transmission positions where it is inserted into small holes or bores in the test material (e.g., the nuclear source is inserted beneath the soil surface, as in a borehole). The gamma radiation received by the radiation detector is related to the density of the test medium by an expression of the following form:CR=Aexp(−BD)−C where:
CR=count ratio (the accumulated photon count normalized to a reference standard photon count for purposes of eliminating long term effects of source decay and electronic drift);
D=density of test specimen; and
A, B, and C are constants.
Nuclear gauges, however, require a high degree of training and radiological management for the operators of these gauges. Therefore, knowing of the desire to obtain accurate field measurement gauges without the use of nuclear gauges, research began in the late 1980s into electromagnetic devices for measuring the density and moisture content of road construction materials such as asphalt, soils and aggregates. These electromagnetic devices have different principles behind moisture and density measurements than their nuclear counterparts.
For moisture measurements in the field, the nuclear device method incorporates neutron moderation, which results in a measurement of the number density of the Hydrogen atoms present in the material. For non-nuclear devices, the moisture measurement is based on the electronic dipole moment per unit volume of the material under test. Most asphalt has a permittivity of less than about 8 and is not terribly dispersive with frequency. Typically, dry soil has an electrical permittivity of about 4, water about 80, and air about 1. In general, soil can have permittivities that range from a dry value of about 4 to a saturated value above 40. In soils, this parameter is frequency dependent. As the percent water increases in sand, soil, aggregate, etc., the dielectric constant increases as well. Therefore, the moisture content can be easily found by measuring the electrical properties of the material. A much more complicated process is required for simultaneous measurement of moisture and density values.
For density measurements in the field, the nuclear device method uses gamma ray scattering properties of the materials. At energies below 1 MeV, the amount of scattering in a material is directly proportional to the number density of electrons. Since the number density of electrons is related to the material density, by measuring the scattering, the material density is found. The electromagnetic device method uses permittivity changes resulting from the decrease in the air void content of an asphalt mix as it is compacted. Therefore, asphalt density can be estimated by measuring the permittivity of the mix.
Asphalt is a heterogeneous mixture of aggregate, binder, and air. Soil is much more complex and is a mixture of aggregate minerals, air, and water. Air has a dielectric constant ∈r of 1.0, whereas dry aggregate and binder dielectric constant ∈r is about 4.0. Water has a dielectric constant ∈r of near 80 depending on the temperature and purity of the water. The dielectric constant or permittivity is represented by a complex number where the real part represents the energy stored and the imaginary part represents the energy loss in the material. For asphalt, as compaction increases and the density increases, the air voids decrease and the dielectric constant increases. As such, asphalt is mostly moisture free and usually has a simple frequency response. For soil, increasing compaction efforts also increases the dielectric constant. However, soil is a complex heterogeneous mixture of air, water, and solid minerals that has a very complex frequency response that complicates the response. As a result, for soil, measurements of the real and imaginary parts of the permittivity are required to separate the moisture from the density effects. Hence, the frequency response of these materials is also of major interest.
For a given soil, the maximum compaction is achieved at specific moisture content. In the laboratory, the density-moisture content relationship for a soil is determined using the industry standard “Modified Proctor Method”, otherwise known as ASTM D 1557 or the “Standard Proctor Method” known as ASTM 698.
For asphalt, in the laboratory there are three methods of designing/analyzing asphalt mixes: (1) The Marshall method; (2) the Hveem method; and (3) the Superpave method. All three methods produce cylindrical asphalt cores for analysis. One of the most important factors is the material density, which is a primary property in the selection of the best mix design. The material density of cylindrical asphalt specimens is determined using dimensional analysis (mass over volume) or variations of water displacement methods as specified in ASTM standards D 1188 and D 2726.
A particular asphalt mix will contain unique aggregate types, textures, binder, and also contain air voids. For instance, the aggregate may be one of limestone or granite, and have proportions of size and texture from passing the 200 sieve to 25 mm. As a result, the base dielectric constant of a high air void mix may be 4 for one mix and 7 for another. Furthermore, the dielectric constant will only change a small percent as the air void content is decreased by compaction. For example, low to high density may range from 4.0 to 4.7 in the real part of the permittivity.
FIG. 1 illustrates different mixing series and how they encompass different “base” dielectric constants for each mix. In FIG. 1, each line represents the entire dielectric range from low to high density of that species or mix. Furthermore, although there are different intercepts for each mix, the slopes are not much different. The mixes shown in FIG. 1 include both granite and limestone aggregates.
As discussed above, the industry has recently become interested in electromagnetic devices for quality testing of pavement materials. Examples of such electromagnetic devices include Model M2701B manufactured by Troxler Electronic Laboratories, Inc., the assignee of the present subject matter, and Model PQI 301 manufactured by TransTech Systems, Inc. Both of these gauges use a single frequency (continuous) source which can be modulated, wherein the M2701B operates at about 50 MHz and the PQI 301 operates at a much lower range frequency. These devices are planar in that they are placed on top of the surface to be measured and fringe a field of energy into the material of interest. Typically, a high frequency electrical signal is passed through the capacitive-sensor placed on the testing material. The signal characteristics measured by electrical signal detection circuitry are then compared with those obtained by placing the sensor on known materials. A correlation to the material density is then used to estimate the density.
Other electromagnetic devices include the TDR as sold by Durham Geo Slope Indicator; ground penetrating radar or GPR; resistive devices such as that marketed by Humbolt and described in U.S. Patent Application Publication No. 2004/0095154; swept or stepped frequency devices such as that manufactured by Greer and described in U.S. Pat. No. 6,388,453; and microwave systems including systems described in U.S. Pat. Nos. 6,316,946 and 5,952,561 and U.S. Patent Application Publication No. 2005/0150278. Electromagnetic devices that operate in the “backscatter” as well as “transmission” mode are also envisioned with bandwidths of several GHz, such as the device described in U.S. Patent Application Publication No. 2005/0150278.
Moisture sensors can be stand-alone versions as well as dual density/moisture probes. Two examples of the stand-alone moisture probes are capacitance monitors for soil moisture as described in U.S. Pat. Nos. 5,260,666 and 4,929,885; each of which is assigned to the assignee of the present subject matter, Troxler Electronic Laboratories, Inc. Another class of electromagnetic moisture probes is manufactured by Hydronix.
Because of variations in manufacturing tolerances, sensing probes of the same design will not necessarily sense exactly the same values. Consequently, each sensing probe must be individually calibrated at the manufacturing factory and as a practical matter the probe should be periodically checked (or recalibrated) to assure that the calibration has been maintained.
For nuclear gauges, calibration is typically conducted using three large and heavy blocks of material of different densities. Typically, these blocks are aluminum (160 lbs/ft3 or PCF), magnesium (110 lbs/ft3), and a mix of aluminum and magnesium (135 lbs/ft3). Other prior art in nuclear calibration devices include shielded capacitance standards as manufactured by Troxler Electronic Laboratories, Inc. and described in U.S. Pat. No. 4,924,173.
Electromagnetic gauges are typically factory calibrated using three large slabs or calibration standards (e.g., of a size 2 foot by 1 foot by 6 inches thick) of varying dielectric constants. Three reference data points are obtained and a least squares fit is applied to the data points for the straight-line equation. It is noted, for example, that the three calibration standards typically span the entire dielectric range shown in FIG. 1. In other words, an offset is usually necessary, but the slopes are going to be close to the expected field value.
Since the electromagnetic density gauges as known in the art are also operated on hot materials in the field, it is possible for the electrical properties of the sensor to change with use and time. Also, any changes in the components in the electrical circuitry of the gauge can lead to drifts in the detected electrical signals. Although the gauges have been designed to minimize these problems, the net effect can be significant to the user. As such, the electromagnetic gauges should periodically be recalibrated in the laboratory.
Electromagnetic gauges are typically recalibrated in the laboratory using bulk homogeneous materials of known electrical properties such as plastic, NYLON, PVC, PLEXIGLAS, and glass, to name a few. These materials are typically in the form of slabs with dimensions 12 inches by 12 inches by 6 inches wherein the weight of the standard can approach 130 pounds or more. Standards can also be calibrated using cylindrical specimens, much like the cores drilled in the field or made in the lab. In any case, the gauge is placed on each material and the signal characteristics are recorded. Using a mathematical model that relates the signal characteristics to the assigned density value, the calibration coefficients are then determined. The systematic errors determined can be corrected in the laboratory using the calibration standards. The measurements taken on the standards will show the changes and if the changes are small enough, adjustment to the calibration coefficients can be made. Hence, confirmation readings obtained on the standards will indicate if recalibration or a simple offset will be necessary. It is additionally known that many regulatory agencies now require that electromagnetic gauges have a reference standard for obtainment of these confirmation readings.
The response of electromagnetic gauges is related to the electrical properties of the material being tested. Therefore, calibration of the devices must typically be performed in the field at regular periodic intervals for determining the density and moisture content of materials. This calibration is typically correlated to known standards. For example, with density gauges, FIGS. 2A and 2B show the comparison of gauge readings of factory-calibrated gauges to true density values for various asphalt mixes (FIG. 2A relates to limestone mixes and FIG. 2B relates to granite mixes). Conversion of the direct reading to the absolute density reading can be performed using cores extracted from the pavement or nondestructively by comparing and correcting using a nuclear method.
For instance, the calibration equation for the M2701B device can be selected for different mix types as found empirically in the field. For example, the operator can do simple offsets or a full-blown slope-intercept calibration to arrive at the calibration equation. This is sometimes achieved using a “test” strip wherein cores are removed from the test strip with different compaction efforts. Other times, a good compaction effort is made and a core is removed and gauge read. The gauge is then offset with one core. Here, for example, the operator would obtain a reading using the M2701B device, remove a core sample, and test it for density properties in the laboratory. Another well-known method is to obtain simultaneous readings using both a nuclear gauge and the electromagnetic device. The nuclear results are used to obtain the calibration in the electromagnetic device. The calibration in the gauge for a simple mix is therefore a simple “y=mx+b” equation or even a “y=a+b*exp(−c*x)” relationship as is currently being explored. A single core or an average of multiple cores can be used for a simple offset. If a good spread of densities is obtainable, then both slope and intercept can be calculated using standard methods.
It is known that electromagnetic devices must also be referenced (calibration confirmed) in the field preferably daily to account for any daily variances encountered. In order to provide this capability in the field, for example, the Model M2701B comes with a “portable” standardization block as shown with reference to FIGS. 3 and 4. As shown in FIG. 3, this standard block S is typically made out three (3) slabs of glass G, each typically having dimensions of 6 inches wide by 6 inches long by 0.5 inches thick, wherein the slabs of glass G are glued together. Typically, a thin layer of FR4 glass epoxy laminate L is overlaid on top of the glass slabs to provide protection to the glass. The standard block S is typically installed with top and bottom layers of foam F in a carrying case C for the gauge device D (see FIG. 4 showing laminate L overlaid on standard block S).
There are several limitations of this prior art standardization block S including: (1) fragility—because of the nature of glass it is possible to damage the block during transport of the gauge, thus leading to erroneous readings; (2) manufacturability—several steps that involve gluing using extremely hot glue are required including one operation gluing the standard to shock absorbing thick foam material, another operation gluing foam to the bottom of the case, and another operation gluing a thin FR4 sheet on the top surface of the glass; (3) cost—the type of special glass used for the standard is expensive; (4) weight—the glass-stack standard alone is about 5 lbs.; (5) size—the standard is only slightly bigger than the gauge sensor head; (6) volume—the glass and foam occupy a large space in the case; (7) stability—due to the weight and cumbersome size of the standard, the glue can loosen over time thereby providing erroneous readings; and (8) accuracy—due to the finite thickness of glass (1.5 inches), the gauge reading shows a small dependence on the type material on which the standard is placed.
Thus, there remains a long-felt need for apparatuses and systems of density gauge emulation for the standardization of electromagnetic gauges in the field using easy to manufacture, low cost, durable materials with improved stability during transportation.